Microelectronic substrates and substrate assemblies typically include a semiconductor material having features, such as transistors and transistor gates, that are linked with conductive lines. One conventional method for forming transistor gates (shown schematically in FIGS. 1A-C) is shallow trench isolation (STI). Referring first to FIG. 1A, a typical STI process includes doping a semiconductor substrate 10 to form an at least partially conductive material 11. An oxide layer 14 is disposed on the conductive material 11, and a nitride layer 15 is disposed on the oxide layer 14. A mask 16 having mask openings 17 is then positioned over the oxide layer 15, and the semiconductor substrate 10 is etched to form apertures 60, shown in FIG. 1B. As shown in FIG. 1C, the apertures 60 are coated with a gate oxide layer 61, and a gate material 62 is disposed adjacent to the gate oxide 61. Accordingly, the gate oxide 61 can electrically isolate adjacent gates. The nitride layer 14 and the oxide layer 15 can then be removed.
One drawback with the STI structure described above with reference to FIGS. 1A-C is that the conductive material 11 has sharp corners 63 (shown in FIGS. 1B and 1C) at the edges of the apertures 60. The sharp corners 63 can emit electromagnetic radiation (generally in the manner of an antenna) which can interfere with the operation of adjacent semiconductor features. One conventional approach to addressing this drawback is to oxidize material at the sharp corners 63 by exposing the semiconductor substrate 10 to a high temperature environment (e.g., about 1050° C.). The oxidized material is then removed (for example, with an etchant) to blunt the corners. One drawback with this approach is that the curvature that can be achieved with a high temperature process may be limited. Another drawback is that the high temperature can damage portions or components of the semiconductor substrate. Still another drawback is that the high-temperature process can be expensive, which can increase the cost of the products formed from the semiconductor substrate.
One conventional technique for removing bulk conductive material from semiconductor substrates includes applying an alternating current to a conductive layer via an intermediate electrolyte to remove portions of the layer. In one arrangement, shown in FIG. 2A, a conventional apparatus 60 includes a first electrode 20a and a second electrode 20b coupled to a current source 21. The first electrode 20a is attached directly to a metallic layer 11a of a semiconductor substrate 10 and the second electrode 20b is at least partially immersed in a liquid electrolyte 31 disposed on the surface of the metallic layer 11a by moving the second electrode downwardly until it contacts the electrolyte 31. A barrier 22 protects the first electrode 20a from direct contact with the electrolyte 31. The current source 21 applies alternating current to the substrate 10 via the electrodes 20a and 20b and the electrolyte 31 to remove conductive material from the conductive layer 11a. The alternating current signal can have a variety of wave forms, such as those disclosed by Frankenthal et al. in a publication entitled “Electroetching of Platinum in the Titanium-Platinum-Gold Metallization on Silicon Integrated Circuits” (Bell Laboratories), incorporated herein in its entirety by reference.
One drawback with the arrangement shown in FIG. 2A is that it may not be possible to remove material from the conductive layer 11a in the region where the first electrode 20a is attached because the barrier 22 prevents the electrolyte 31 from contacting the substrate 10 in this region. Alternatively, if the first electrode 20a contacts the electrolyte in this region, the electrolytic process can degrade the first electrode 20a. Still a further drawback is that the electrolytic process may not uniformly remove material from the substrate 10. For example, “islands” of residual conductive material having no direct electrical connection to the first electrode 20a may develop in the conductive layer 11a. The residual conductive material can interfere with the formation and/or operation of the conductive lines, and it may be difficult or impossible to remove with the electrolytic process unless the first electrode 20a is repositioned to be coupled to such “islands.”
One approach to addressing some of the foregoing drawbacks is to attach a plurality of first electrodes 20a around the periphery of the substrate 10 to increase the uniformity with which the conductive material is removed. However, islands of conductive material may still remain despite the additional first electrodes 20a. Another approach is to form the electrodes 20a and 20b from an inert material, such as carbon, and remove the barrier 22 to increase the area of the conductive layer 11a in contact with the electrolyte 31. However, such inert electrodes may not be as effective as more reactive electrodes at removing the conductive material, and the inert electrodes may still leave residual conductive material on the substrate 10.
FIG. 2B shows still another approach to addressing some of the foregoing drawbacks in which two substrates 10 are partially immersed in a vessel 30 containing the electrolyte 31. The first electrode 20a is attached to one substrate 10 and the second electrode 20b is attached to the other substrate 10. An advantage of this approach is that the electrodes 20a and 20b do not contact the electrolyte. However, islands of conductive material may still remain after the electrolytic process is complete, and it may be difficult to remove conductive material from the points at which the electrodes 20a and 20b are attached to the substrates 10.